1. Field of the Invention
This invention relates to an ultrasonic transducer and to a method of manufacturing the same.
2. Description of the Prior Art
Ultrasonic transducers are widely employed as the probes in ultrasound diagnostic equipment for real-time observation of the tomograph of a living body, in ultrasound materials testing equipment for the non-destructive testing of materials, and in many other applications.
Various materials capable of being used as a vibrator in such ultrasonic transducers have been the object of research. One material recently proposed for such use is a piezoelectric polymer material such as polyvinylidene fluoride (PVDF) or a composite of a ferro-electric ceramic powder and a polymer. The reason for this is that a piezoelectric polymer material exhibits excellent acoustic matching with respect to a living body since its acoustic impedence is closer to that of a living body than is the acoustic impedence of conventional ceramic materials. Such a piezoelectric polymer material also has a low mechanical Q, as a result of which improved sensitivity and response are anticipated, and exhibits flexibility that enables the vibrator to be machined into almost any shape with comparative ease. Another reason for using a piezoelectric polymer material is its high anisotropy. Specifically, the vibrating efficiency of such a material in the direction of its thickness is considerably higher than in a direction perpendicular thereto, namely the transverse direction, so that the material can be formed into a vibrator approximating an ideal piston acoustic source. Since the thickness of such a vibrator can be made very small, application in the high-frequency region is widespread.
However, certain problems are encountered when attempting to employ a piezoelectric polymer material to construct a linear or sector-type array of transducers for high-frequency use. Specifically, it is necessary to make the spacing between adjacent elements of the array very small in the width direction of the elements in order to minimize the grating lobe. Since a piezoelectric polymer material constitutes a vibrator approximating a piston acoustic source, as mentioned above, the method most widely adopted to form the array elements is to form only the electrodes into an array pattern, as opposed to the method used with conventional ceramic elements where the elements are formed by cutting them individually with a diamond cutter or the like. In other words, with a piezoelectric polymer material, the array pattern electrodes are formed by a technique such as photoetching, screen printing or vapor deposition. However, when the array element spacing is further reduced for the purpose of minimizing the grating lobe, it is sometimes difficult to form array pattern electrodes which are stable in terms of shape by using the foregoing method. This can lead to a deterioration in acoustic characteristics.